Pressure Die Assist and Outside-Wall Tensile Strain

Outside-wall thinning and wall necking occur when the tensile strain on the tube extrados exceeds the plastic deformation limit of the material during rotary draw bending. As the bend die rotates, the outer wall is stretched while resisting drag forces from the pressure die and wiper die. When bending tight center-line radii or thin-wall tubing, passive pressure die friction increases tensile load, leading to excessive wall reduction or complete tensile failure along the outer radius. Pressure die assist applies positive forward thrust directly to the pressure die body, pushing material toward the bend zone to relieve tensile stress and maintain wall thickness integrity.

Tensile strain mechanism on the bend extrados

During rotary draw bending, the neutral axis shifts toward the inside radius of the bend. This shift concentrates plastic deformation on the outside wall, where the material must stretch to cover the larger arc length of the outer radius. Without active force intervention, the tension required to draw the tube around the bend die comes entirely from the clamp die torque. This tension pulls material through the bending tooling, generating significant friction drag against stationary or passive surfaces.

Higher drag forces increase tensile stress forward of the tangent point. If the wall thickness ratio relative to tube diameter is low, or if the center-line radius is tight, the outer wall thins beyond acceptable manufacturing tolerances. Severe tensile strain also results in cross-sectional ovality as the unsupported outer wall collapses inward toward the center line. Applying forward pressure through the pressure die reduces the tensile load transmitted from the clamp die, transferring a portion of the axial force needed for deformation to the pressure die actuator or servo drive.

Setting assist pressure and push speed

To control wall reduction without causing secondary defects, pressure die assist force must stay closely synchronized with the linear advancement of the tube as it is drawn around the bend die. If the assist speed is slower than the tube surface speed, the pressure die acts as a drag brake, increasing extrados necking and surface scoring. Conversely, if assist boost pressure or advance speed is set too high, material is driven aggressively into the bend zone, which increases the risk of inside-radius wrinkling, heavy pressure die marking, or severe clamp die slippage.

In standard setups on the CNC Boru Bükme Makinesi (Griffin), assist pressure is regulated based on tube yield strength, wall thickness, and bend geometry. Boost speed is programmed as a percentage match to the bend die angular velocity. On thin-wall stainless steel or aluminum alloys, initial setup starts with boost speed matched to tangent speed, while hydraulic or servo boost pressure is adjusted in incremental steps until wall thinning remains within specified limits without generating compression ripples at the tangent entry.

Identifying pressure die slip and drag marks

Visual inspection of the tube outer surface and pressure die groove provides direct feedback on force balance during the bend cycle. When pressure die assist force is insufficient, distinct drag marks or metallic pickup appear along the contact length of the pressure die. This condition is accompanied by measurable wall thinning exceeding calculated strain limits. This is one of the first things we check when a customer reports extrados cracking during high-angle bends.

Excessive boost force presents the opposite physical signature. If the pressure die pushes faster than the draw speed, material accumulates ahead of the bend die tangent point, producing a raised ridge or micro-wrinkle on the inside radius prior to full die contact. Severe over-boost can also cause the tube to slip within the clamp die insert, leaving noticeable galling or step marks at the clamp transition point. Observing tool marks along the pressure die trace indicates whether friction drag or over-thrust dominates the bend process.

Verifying force balance in the ETU control

Modern CNC tube benders, such as the Tam Elektrikli Boru Bükme Makinesi (EOS), utilize real-time axis monitoring to regulate pressure die assist positioning and boost thrust. Within the ETU control software, assist movement is mapped to degree position rather than time-based intervals. This ensures that thrust remains relative to the bend angle regardless of machine cycle speed variations.

During setup, process engineers review force feedback curves and axis position tracking in ETU to ensure the pressure die advances smoothly without hydraulic lag or servo positioning errors. Key parameter checks in ETU include:

• Booster start angle and drop-off position relative to bend die rotation.
• Assigned boost force percentage relative to material yield strength.
• Pressure die unclamping setback distance at cycle completion.
• Servo torque limits to prevent tube buckling during initial clamp engagement.

Adjusting these position-based parameters allows precise control over wall thinning while protecting tooling surfaces from premature wear caused by excessive side loads.

To evaluate pressure die assist requirements for your bending applications, submit your 3D STEP files or scaled technical drawings along with material grade, tube wall thickness, center-line radius, and estimated annual production volume to our engineering team.